The present invention generally relates to a method and apparatus for monitoring a polishing condition of a surface of a wafer in a polishing process. The present invention is particularly useful for determining an end-point in a chemical mechanical polishing (CMP) process.
Chemical mechanical polishing (also referred to as chemical mechanical planarization) or CMP is a proven process in the manufacture of advanced integrated circuits. CMP is used in almost all stages of semiconductor device fabrication. Chemical mechanical planarization allows the creation of finer structures via local planarization and for global wafer planarization to produce high density structures.
During a CMP process, a substrate is mounted to a carrier or polishing head. The exposed surface of the substrate is moved against a rotating polishing pad on a polishing platen. A polishing slurry is distributed over the polishing pad. The slurry includes an abrasive and at least one chemically reactive agent. The abrasive chemical solution is provided at the interface between the polishing pad and the wafer in order to facilitate the polishing.
It is generally desirable to control the CMP process to find an endpoint for polishing or to determine the thickness of a polished layer.
One prior art attempt to control the CMP process uses pre and/or post measurements of wafers with either manual or automatic processing. Systems are available which allow measurement of the wafers immediately before and after polishing. If the film thickness before and after polishing is known, it is possible to adjust the polishing parameters and to optimize the polishing process within a production sequence. However, such a pre and/or post measurement method has the disadvantage that at least the first wafer or the first few wafers have to be polished with the default parameter settings, i.e. without optimized parameters. Typically, these first wafers are targeted to underpolish, such that subsequent repolishing can be done to achieve the specification range.
Several methods have been suggested to obtain a reliable endpoint for the polishing process. Current methods include measuring temperature, shaft friction, vibration, sonic level, or frequency. Unfortunately, these methods do not work for all substrates, particularly when an oxide is polished. A large number of CMP processes use timed polishing steps for specific films or wafers. These processes generally lead to a relatively wide range of results, as the variation of factors such as polish head condition, slurry refreshing, down force, or pressure cause the polishing rate to change during the processing of a large batch of wafers.
Since overpolishing of wafers is catastrophic and severe overpolish may result in destroyed wafers, wafers are typically targeted to underpolish, since an under-polish condition may be removed by reprocessing the wafers to bring them up to specification. However targeting for an underpolish often leads to a significant number of wafers that require repolishing, thereby lowering the throughput and increasing the overall processing costs. Further, the time for which the underpolished wafers need to be repolished is usually calculated manually, taking the removed film thickness, the target thickness and the wafer polish time into account. Repolishing thus requires significant human resources.
While for larger device dimensions the process target specifications tend to be rather relaxed, there are increasing requirements to tighten the film removal range as device technologies shrink.
In view of the above, the present invention seeks to solve the above mentioned problems and shortcomings of the prior art and intends to provide a method and an apparatus which allows for an improved determination of the endpoint in a polishing process.
It would further be advantageous to have a method for polishing wafers with increased throughput, improved process uniformity and reduced processing costs.